Pay attention to the spacing and direction of your pins!
As you can see in the image above, the piece on the right is pinned both infrequently and also with the pins perpendicular to the knitting. As the piece dries, the pins hold only the pinned portion of the knitting in place, causing scallops where the fabric should be straight. This leads not only to warped stitches, but makes finishing more difficult. On the left, though, not only are more pins used but they are all positioned in the same direction as the edge of the fabric; on the top of the piece, they are pinned horizontally, and on the sides, they are pinned vertically. This way, the full edge of the pin is holding the edge of the fabric in place, and as it dries, the edge stays nice and straight. And that's it: a nice quick little tip that doesn't actually take any additional time, but really elevates your finishing!